The present invention relates generally to the field of bearings and the lubrication of such bearings. More particularly, the invention relates to a novel arrangement for retaining lubricant in a bearing lubrication system upon shutdown of the system.
Wide varieties of bearings and other power transmission components are available and are currently in use throughout a range of industrial applications. Bearings are generally used for facilitation of rotational movement in a mechanical application, and typically include a plurality of bearing elements situated in a housing. Depending upon the application and the anticipated loading, the bearing elements may be journal bearings, needle bearings, roller bearings, ball bearings, and so forth. Moreover, lubrication of the bearings generally prolongs the useful life by reducing corrosion and wear, and by providing for sealing and cooling of the bearing.
Lubrication may reduce friction between components and between a bearing and a rotating shaft, such as through the formation of a thin film of lubricant between the components. Lubrication may also reduce friction between the bearing elements, for example, between inner and outer rings or races. The lubrication may also aid in carrying away contaminants or small debris which may find their way into the bearing or which may be released from the component parts over time, and may serve to substantially preclude the ingress of contaminants (e.g., dirt, debris, moisture and so forth) into the bearing. As for the type of lubricant, bearings may be lubricated with grease, oil, a solid lubricant, and so forth, with the choice depending on factors such as the temperature range, operating speeds, loading conditions, economics, and the like. Moreover, lubricants may be changed from time to time because their properties may deteriorate as a result of aging and contamination of the lubricants. As for oil lubrication systems, the various system types may include an oil bath, dripping oil, circulating oil, oil jet, oil mist, and so forth.
Employment of a circulating oil system, though potentially increasing initial investment costs compared to greased systems and other oil systems, is generally an effective means of removing heat from a bearing by providing an adequate continuous flow of cool, clean oil, and offers advantages including improved dissipation of heat and effective flushing of contaminants, such as dirt, dust, moisture and wear metals. In general, a circulating oil system may take a lubricant, such as mineral oil, from a tank or other source, and pump the oil through supply piping to one or more bearings, and return the oil through return piping to the tank. Pumping of the oil may be accomplished, for example, with a positive displacement pump, such as a gear pump, which may be coupled to the oil source (e.g., oil tank). Exemplary piping elements in the supply and return piping may include tubing and tubing connections, carbon steel and stainless steel piping, screwed and flanged piping connections, other fittings, flow devices, and the like.
Further, circulating oil systems may employ a variety of features, such as instrumentation (e.g., pressure and temperature indication), filtration, thermal processing, and so forth. Filtration (e.g., installed in the supply line) may be utilized to maintain the lubricant in a useable condition for a longer period of time, and to reduce the amount of contaminants introduced (or re-introduced) to the bearing elements. Additionally, thermal processing through the use of a heat exchanger, for example, may control the temperature of the oil or lubricant to prevent thermal damage to the lubricant, the rotating shaft and/or the bearing elements. In sum, circulating oil systems may vary widely as to detail, the particulars of each design depending on the make, size, type, location and purpose of the oil system and lubricated bearing. Moreover, system and equipment design may depend on factors, such as projects economics, operating conditions, design standards, and so forth.
For instance, the size of an oil reservoir or tank, which may range from a few gallons to several thousand gallons, may be based on a variety of design and operating factors of the circulating oil system. The tank capacity may be based on the volume or rate of the oil circulated. For example, the tank capacity in gallons may be designed to equal a factor (e.g., 5-10 times) the gallons of oil circulated per minute. In another example, the tank capacity may be sized to provide a minimum retention time (e.g., 2-5 minutes) between the pump suction and the minimum operating level in the tank. Many other examples of design bases for the size of the oil tank or reservoir exist in the art. Moreover, to reduce the number of different product offerings, manufacturers and users may standardize on one size of reservoirs which may hold more, or in some cases considerably less oil, than the volume in circulation between the system and the serviced bearings.
A special case is where the tank capacity is sized to accommodate draining of the oil from the supply and return piping (upon shutdown of the oil circulation pump, for example). In other words, the tank capacity may be sized to exceed the volume of the supply and return piping, and thus exceed the volume of oil contained in the piping in a hydraulically full system. However, for long runs of supply and return piping, the designed tank size may increase to the point where the layout or handling of the tank becomes cumbersome (e.g., difficult to install on a small skid unit), or where the cost of the tank becomes prohibitive. Furthermore, design of a tank capacity based on the piping volume is not applicable where the piping layout is not known prior to the design and fabrication of the oil tank. Accordingly, other means are needed to accommodate the oil in the piping and to prevent, for example, overflow of the oil tank when the oil circulation pump is shutdown and the oil in the supply/return piping drains (i.e., by gravity) back to the tank.
For tanks with a relatively small volume or a high normal operating level (small vapor space), the oil that drains from the piping may overfill the tank, especially with long runs of supply and return piping. As the tank overfills, the oil may leak from the tank (e.g., through the tank breather) and may create an oil flood on the surrounding ground or floor, as well as cause loss of oil in the total system. Such loss can lead to emptying of the tank and bearing sump, which can then adversely affect the operation of the pump and the bearings. As indicated, increasing the size of the oil tank significantly such that any oil flowing back will not cause over filling of the tank requires prior knowledge of the size and length of the connecting pipes, which is often not available. Moreover, even where the piping data is available, the size of the tank may become cumbersome or cost prohibitive, and requires multiple product offerings. Also, it should be noted that it may be desirable to avoid draining of oil from the piping and bearing even if the tank is large enough to accommodate the system oil. For example, instead of draining oil from the piping and bearing to the tank, it may be beneficial to maintain the oil in the bearing sump or housing for static lubrication so that the bearing may continue to operate while the oil circulation is shutdown.
There is a need, therefore, for an improved technique for circulating lubricating oil through a bearing, while effectively managing the shutdown of the system. In particular, there is a need for an improved technique to accommodate the oil contained in the supply and return piping upon depressuring of the oil circuit (e.g., shutdown of the circulation pump) to prevent overflow of the oil source (tank). Also, there is a need to retain the oil in the bearing housing to provide for static lubrication upon shutdown of the oil circulation.